Chemical and Physical Characterisation of Tyre Wear Particles in the Environment

The release of tyre wear particles (TWP) from vehicular tyres has been gaining global attention as a significant contributor to microplastic contamination of the environment. The frictional interaction between the tyres and road surfaces results in the wear and tear of the tread which contributes to global emissions of an estimated 6 million tonnes of TWP. However, there is a knowledge gap about the environmental availability of materials derived from tyre particles due to the challenge in identifying the particles in complex matrices and a subsequent lack of standardisation in the analytical methodologies for TWP analysis. At this stage, the potential of cryogenic grinding (cryomilling) has been explored to produce in-house tyre particles of known physical and chemical characteristics. A microscopy method was developed and applied to measure the size of cryomilled tyre particles (CTP) produced at varying cryomilling conditions to determine the optimum milling parameters. At maximum milling time and speed, a mean particle size of 50 ± 42 µm was achieved but, particles below 10 µm were also produced and identified. Thermogravimetric analysis (TGA) profiles of CTP showed that more than 50% of the weight loss is attributed to the total rubber content and that the carbon black content varied significantly amongst the tyres produced by four manufacturers. These profiles could also be used to deduce the type of tyre based on the difference in the rubber content. Fourier transformed infrared (FT-IR) spectroscopy of CTP showed tyres produce a specific spectrum which could be used as a fingerprint and the slight variations in the bands at 1700 and < 1000 cm^-1 could be due to various additives. Surface area and charge determination of CTP are the next steps to provide a full characterisation of the tyre particles produced via cryogenic grinding which, will aid future analytical method developments and sorption experiments in this research.